1. Field of the Invention
This invention relates to the coating of a traveling paper web. More specifically, this invention relates to apparatus for controlling the depth and turbulence of the pool of liquid coating material in the trough between two nipped rolls in a roll-type coater. Still more specifically, this invention relates to a unique design of an end deckle having one or more openings which permit the outward flow of liquid coating material through the deckle as a function of the depth of the material in the trough between the nipped rolls.
2. Description of the Prior Art
Roll-type coaters, which utilize a plurality of nipped rolls to meter coating material applied to either the surface of one or more of the rolls or to a trough formed between nipped rolls, have been known for a long time. Also known is the use of end deckles in coaters to form a trough with the surfaces of the rolls to maintain a pool of liquid coating material between the nipped rolls. Sometimes, a paper web is run through a pool of the coating material in a trough between nipped rolls, and sometimes the web is run between other nipped rolls while the trough supplies coating material to the surface of a roll which is, in turn, nipped with a roll which nips the web such that the coating material is metered before contacting the web.
As long as the pool of coating material is relatively viscous, and the speeds of the nipped rolls forming the trough for the pool of coating material are relatively slow, or the pool of coating material is relatively shallow, or some combination of these operating conditions are present, problems regarding coating material pool turbulence and skipping of the coating material at certain locations on the web are not significant or can be controlled by simply supplying more coating material to the trough. However, even having deeper pools in the troughs does not necessarily solve all the problems as machine speeds are increased. A pool of coating material has two interfaces with the downwardly rotating metering rolls. The coating liquid at and near the interfaces tends to travel in the downwardly direction with the roll surfaces. Since all of the interface portion of the pool cannot pass through the nip, some of the coating liquid is forced back upwardly in the inner portion of the pool to erupt from the surface and splash about.
This problem could be alleviated by slowing the machine speed. However, to be economically competitive, the coating operation, like other aspects of papermaking, has to be done faster than before, but with the same or improved quality. In addition, more types of paper are being coated than ever before, and there are more lighter grade papers being coated with less viscous, almost water-like coatings, such as starch used to coat newsprint to reduce subsequent dusting problems in the printing operations.
If less viscous coating compositions are used in prior roll coating apparatus and the coating apparatus is operated at higher speeds, or if the depth of the pool of coating material is increased, or some combination of these operating parameters are present, the pool of coating liquid becomes increasingly agitated and turbulence develops. The turbulence in the coating pool can become so great that the coating material begins to erupt and shift, which has several undesirable consequences, such as wasting the heated coating material; splashing on previously coated paper or uncoated paper; creating such deep depressions in the pool that streaks of uncoated, or non-uniformly coated areas of the paper web develop. Naturally, none of these results are desirable, but some of them produce defects in the paper so great that the paper can either not be sold, or must be sold at a discounted price. Some grades which must be of superior quality are papers intended to be laser printed and computer paper grades.